The present invention relates to a method of coordinating measures influencing the driving performance of a vehicle. The present invention also relates to a device for coordinating measures influencing the driving performance of a vehicle.
Various systems are known through which the driving performance of a vehicle is influenced automatically. Known systems include, for example, the anti-lock braking system (ABS) and the traction control system (TCS) which support the driver in braking and acceleration in situations that are critical from the standpoint of longitudinal dynamics. Likewise, there are also known systems which are helpful in situations that are critical from the standpoint of transverse dynamics. For example, driving dynamics regulation (FDR) and the electronic stability program (ESP) support the driver in situations that are critical from the standpoint of both longitudinal and transverse dynamics.
Furthermore, there are known systems which influence the driving performance of a vehicle through measures affecting the steering system (xe2x80x9cdynamic steering systemxe2x80x9d (FDL)). Such a dynamic steering system also stabilizes driving performance in addition to having comfort and handling advantages, with stabilizing measures being implemented in addition to measures influencing the vehicle through other systems such as ESP. In this case, superpositioning of an additional steering angle on the front wheels is used to achieve a more comfortable stabilization (by using the measured yaw rate, a calculated yaw rate setpoint, and suitable regulator algorithms) than is possible through the ESP braking measure. Such an additional stabilization through the dynamic steering system offers special advantages, especially at high coefficients of friction.
However, in coordinating the various measures, determining suitable parameters on the basis of which it is possible to decide which measure should be taken at which time to have the best effects on the driving performance of the vehicle is problematical.
The present invention is based on the method that, due to the fact that the wheel slip angle is compared with the threshold value for the wheel slip angle, an intervention in the braking system and/or the drive system of the vehicle is implemented at an absolute wheel slip angle greater than the absolute threshold value of the wheel slip angle. Above a certain wheel slip angle, any further increase in wheel slip angle always results in progressively smaller increases in adhesion, so that regulation of the wheel slip angle by a dynamic steering system is no longer appropriate above the threshold value of the wheel slip angle. Instead, it is beneficial to perform an intervention through the brake system and/or the drive system of the vehicle, e.g., as part of ESP.
At an absolute wheel slip angle which is smaller than the absolute threshold value of the wheel slip angle, preferably no intervention is implemented through the brake system and/or the drive system of the vehicle, and instead a measure is implemented through the dynamic steering system of the vehicle. Thus the intervention measures through ESP and the dynamic steering system, for example, may be separated, and in particular no ESP intervention is performed if the regulation can be performed on the basis of the dynamic steering system alone.
It is advantageous if the wheel slip angle of the front axle is estimated on the basis of the equation       α    v    =      β    +                            v          Gi                ·                  l          v                            v        Fzref              -    δ  
where
xcex1vis the wheel slip angle
xcex2 is the float angle (attitude angle)
vGi is the yaw rate of the vehicle
vFzref is the vehicle velocity
xcex4 is the steering angle at the front axle.
The yaw rate of the vehicle may preferably be measured, while the float angle and the vehicle velocity may be estimated. Thus, a wheel slip angle xcex1v may be determined from quantities already available as part of related art systems.
It may be especially advantageous in this connection if the float angle is estimated by integration of the equation             ∂              β        ⁡                  (          t          )                            ∂      t        =                    a        y                    v        Fzref              -          v      Gi      
where
xcex2 is the float angle
ay is the transverse acceleration of the vehicle
vFzref is the velocity of the vehicle
vGi is the yaw rate of the vehicle.
Again, the yaw rate of the vehicle may be a measured quantity, and the velocity of the vehicle may be estimated. The transverse acceleration of the vehicle may also be a measured quantity. Integration of the given equation may be performed by using special algorithms and simplifying assumptions so that ultimately a vehicle float angle signal xcex2(t) may be obtained.
It may be especially advantageous if the coefficient of friction on the front axle is estimated on the basis of the equation
xcexc={square root over (ax2+ay2)}/g
where
xcexc is the coefficient of friction
ax is the longitudinal acceleration of the vehicle
ay is the transverse acceleration of the vehicle
g is the acceleration due to gravity.
Acceleration due to gravity may be assumed to be equal to 9.81 m/s2. The transverse acceleration of the vehicle may be measured, and the longitudinal acceleration may ultimately be estimated using the equation
ax=∂xcexdFzref/∂t.
It may thus be possible to predict tire performance, so that ultimately it may be possible to estimate the threshold value of the wheel slip angle which is used as a criterion for employing the different measures intervening in the vehicle dynamics.
It may be especially advantageous that the wheel slip angle threshold value is determined as the point of intersection of two straight lines, the two lines forming tangents to the xe2x80x9cadhesion vs. wheel slip anglexe2x80x9d function. The first straight line may be a tangent to the point of origin and the second straight line may be a tangent to the saturation curve at a large wheel slip angle. The typical curve for xe2x80x9cadhesion vs. wheel slip anglexe2x80x9d at first has a constant slope of the adhesion curve at a low wheel slip angle which then flattens out at an increasing wheel slip angle and then reaches saturation. A decline in adhesion can be expected with a further increase in wheel slip angle. If one takes a straight line which forms a tangent to the function at the point of origin and also a straight line which is a tangent to the saturation range, the point of intersection of these two lines yields a wheel slip angle which is a function of the coefficient of friction. In the case of wheel slip angles larger than the wheel slip angle threshold value thus determined, an increase in the wheel slip angle results in weaker and weaker increases in adhesion.
It may be especially advantageous if, in the case of a dynamic steering system which is not active, the wheel slip angle threshold value is set at zero. In this case, activation of an intervention measure into the brake system and/or the drive system of the vehicle is made possible in every case, regardless of the prevailing coefficient of friction.
The present invention provides means for comparing the wheel slip angle with the wheel slip angle threshold value, and in the case of an absolute wheel slip angle being larger than the absolute wheel slip angle threshold value, an intervention measure is taken through the brake system and/or the drive system of the vehicle. A further increase in the wheel slip angle beyond a certain wheel slip angle leads to progressively smaller increases in adhesion, so that regulation of the wheel slip angle based on a dynamic steering system is no longer appropriate above the wheel slip angle threshold value. Instead it is beneficial to intervene through the brake system and/or the drive system of the vehicle, e.g., as part of ESP.
The present invention provides great comfort advantages obtained through the separation of the intervention measures between the brake system or the drive system of the vehicle (e.g., on the basis of ESP) and intervention measures into a dynamic steering system. Below a wheel slip angle threshold value, ESP does not usually intervene, whereas the ESP is triggered when the wheel slip angle is above the wheel slip angle threshold value, because influencing the steering angle would no longer have an effective influence on the driving performance.